NASA's Blazing the Trail to Further Understand Comets

For the first few decades of the space program a majority of
researchers devoted their attention to the planets and their
satellites or moons. These planets and satellites were large and
obviously fascinating and could be compared easily to our own Earth.
Geochemists and astronomers pointed out that these large celestial
bodies have greatly evolved over the eons, modified by high
temperatures, great pressures, and the forces of chemical reactions
and physical erosion. Knowledge of the earliest days of the solar
system could only be gained by studying the small bodies within the
system such as comets, asteroids, and meteoroids, which were little
changed over the past 4 1/2 billion years.

Halley's Comet shown in the Bayeux Tapestry

Comets have always fascinated mankind because of their unique and
strange behavior, appearing unexpectedly, persisting only briefly as
a changing object of considerable beauty, and then rapidly
disappearing. In medieval times they were often feared as harbingers of
catastrophe. Many were afraid that these "serpents" would eat the
Earth and destroy mankind. Edmond Halley first showed that comets
were a natural part of our solar system by successfully predicted the
return of the comet later named after him. Living astronomer, Fred
Whipple, in papers published in 1950, first suggested the true nature
of comets. The entire cometary phenomenon, with a tail that can
stretch over a hundred million miles, originates in the small icy
conglomerate of dust and frozen gases, only a kilometer to a few tens
of kilometers in diameter, called the nucleus. The visible comet is
all a temporary atmosphere of dust and gas escaping from the nucleus
and caused by solar heating and radiation.

Image of Comet Hale-Bopp taken by Wally Pacholka on April 5, 1997 from
the Joshua Tree National Park in California.

Since comet nuclei are so small, it is difficult to study them with
Earth-based telescopes. Following the pioneering efforts of the past
space missions such as ICE, Giotto, VeGa, and the Suisei spacecraft
and their studies of comets Giacobini-Zinner, Halley, and
Grigg-Skjellerup, a series of efforts began to study both the
operational and cosmological significance of comets. NASA developed a
technology spacecraft, Deep Space 1 (DS1) which flew past P/Borrelly
in mid-2001 after completing its technology evaluations. The Stardust
mission, launched on February 7, 1999, will make the first attempt at
serious cosmological study by bringing back a sample of cometary dust
(from an encounter with P/Wild 2 on January 2, 2004) to Earth on
January 15, 2006. Using the full panoply of instruments available in
Earth laboratories, from electron microscopes to particle
accelerators, scientist will be able to determine the isotopic and
mineralogical composition of comets and the temperature and pressure
conditions in the early solar system. On the operational side, the nucleus of
P/Wild 2 will be imaged and the amount, size, and atomic composition
of the dust, that flows from it, determined by two other instruments.
Other small body spacecraft under construction include Contour, Deep
Impact, Rosetta, and the recently awarded Discovery mission, DAWN.
The next decade should bring great advances in mankind's knowledge
of both small bodies and the solar system.